Agricultural Autopilot Path Adjustment

ABSTRACT

Predictive tractor path adjustments improve implement tracking performance by enabling agricultural autopilots to anticipate the effect of curves, slopes, changing soil conditions and other influences.

RELATED APPLICATIONS

This application is a division of U.S. Ser. No. 13/108,826 filed on May16, 2011.

TECHNICAL FIELD

The disclosure is related to autopilots for agricultural equipment.

BACKGROUND

Farmers in the United States operate over two million farms coveringroughly one billion acres of land and producing hundreds of billions ofdollars of crops each year. The farmers spend tens of billions ofdollars per year on seeds, fertilizer, chemicals and fuel. A modern farmis a complex operation where precision and efficiency can have asignificant impact on the bottom line. According to the USDA, the mostefficient 25% of US corn growers spend about $1 to grow a bushel of cornwhile growers in the least efficient 25% spend $3 to grow the sameamount.

One way farmers improve efficiency is by avoiding unnecessary overlapsin tilling, spraying and harvesting operations. In other words, theyavoid driving their tractors and equipment over the same area twice.Consider an 80-acre field and a 44-foot wide sprayer towed behind atractor as an example. The sprayer is towed across the field in seriesof overlapping tracks. If the overlap between adjacent sprayer tracks isreduced from two feet to four inches, then four acres of spraying areeliminated. Such precision may be achieved by guiding tractors withglobal positioning system (GPS) based steering systems.

Precision control of towed farm implements such as plows, rippers,disks, planters, applicators, drills and other equipment has otherbenefits as well. It makes it easier to operate machinery in dark ordusty conditions. Operators can drive faster and reduce driving stress.The quantity of fuel and chemicals used can be decreased, thereby savingmoney and the environment. Soil compaction can be avoided by keepingheavy equipment on precise tracks.

Advances in GPS technology (and systems based on other globalnavigational satellite systems (GNSS) such as the Russian GLONASS andthe European GALILEO) have made it possible to drive large farm tractorsalong predetermined paths very accurately. A tractor can return to afield a year after first working it and follow the same track within aninch. Control of towed implements is more difficult, however.

A towed implement is attached to a tractor by a hitch and the tractorpulls the implement across the ground. The implement may wander off itsintended path for any number of reasons including asymmetrical loading(e.g. tougher ground to plow on one side than the other) or drag due tooperating on a slope. Skilled tractor operators can compensate for awandering implement by deliberately steering the tractor away from adesired path so that the implement stays on the path even though thetractor does not. However, despite the best efforts of operators, thismanual method is imprecise, takes a long time and travel distance, andcauses operator fatigue.

“Automatic Control of Passive, Towed Implements” (U.S. patentapplication Ser. No. 12/244,198, filed on Oct. 2, 2008) describessystems and methods that guide towed implements along a desired path bydirecting an autopilot-controlled tractor optimally off the path. Theseand other feedback methods for controlling towed implements provideoptimal reactions to disturbances. They decrease the response timerequired for an implement to execute a step offset from a predeterminedpath.

Despite the success of feedback control systems for towed implements,further improvements are possible. In particular, feedback controlsystems have not eliminated implement tracking errors that can occur oncurved paths or changing slopes. What is needed are new systems andmethods to ensure that towed implements follow their intended paths asaccurately as possible even when those paths include curves, changingslopes or other known influences.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a tractor towing an implement along a desired path.

FIG. 2 illustrates equidistant swath layout.

FIG. 3 illustrates a basic curved-path adjustment.

FIGS. 4A and 4B illustrate an advanced curved-path adjustment.

FIGS. 5A and 5B illustrate a sloped-path adjustment.

FIGS. 6A and 6B are graphs of implement sag versus slope and coefficientof friction, respectively.

FIGS. 7A and 7B illustrate a crosswind-path adjustment.

FIG. 8 provides an overview of an agricultural autopilot that makespredictive path adjustments.

FIG. 9 is a flow chart for a predictive path adjustment method.

DETAILED DESCRIPTION

Tractors pull implements. But implements do not follow tractors' pathsexactly. To make an implement follow a desired path, a tractor often hasto take a different path.

Conventional feedback control systems for towed implements react todisturbances. As soon as an implement starts to drift away from itspath, the control system guides a tractor to pull the implement back ontrack. The systems and methods described below improve implement controlperformance by anticipating the effects of influences such as curves,slopes, soil changes and wind.

Advance knowledge of influences may be obtained from maps of desiredimplement paths, topography, soil conditions, wind and other factors.Maps and other data may come from external sources or may be measuredfor later use.

Given a desired implement path and anticipated influences, an optimaltractor path may be predicted. In the absence of disturbances, a tractortravelling along its optimal path tows an implement along its desiredpath and the effect of anticipated influences is eliminated. Forexample, the anticipated effect of a known slope (implement slipsdownhill) is taken care of in advance by a tractor path that lies uphillfrom the desired implement path. Feedback control is still useful tocorrect for disturbances, such as bumps and unanticipated influences(e.g. an unmapped hill), for these can never be completely eliminated.

FIG. 1 illustrates a tractor towing an implement along a desired path.Tractor 100 pulls implement 110 via hitch 105. Dashed line 115 indicatesa desired path along which the implement is to move. Note that althoughthe implement is on desired path 115, the tractor is not. The positionof tractor 100, as represented by a reference point at the midpoint ofthe tractor's rear axle, is measured by a GNSS receiver connected toantenna 120. Similarly, the position of implement 110 is measured by aGNSS receiver connected to antenna 125 located at a reference point onthe implement. In practice, antennas 120, 125 are usually mounted abovetheir respective reference points. When the tractor and implement pitchand roll away from level, the antennas are no longer are no longer atthe same horizontal position as the reference points. Inertialmeasurement units (IMU) may be used to measure pitch and roll of thetractor and/or implement and the pitch and roll angles may then be usedto correct for antenna lever arm errors. When IMU is used with GNSS, thecombination is referred to as INS, for “inertial navigation system”.

(The GNSS receivers used to measure the position of the tractor and theimplement may take advantage of corrections such as those provided bysatellite or ground based augmentation systems (SBAS or GBAS). Examplesof SBAS include the Federal Aviation Administration's Wide AreaAugmentation System (FAA WAAS), the European Geostationary NavigationOverlay Service (EGNOS) operated by the European Space Agency, theMulti-functional Satellite Augmentation System (MSAS) operated byJapan's Ministry of Land, Infrastructure and Transport, and variousproprietary systems operated by commercial enterprises. Examples of GBASinclude the United States' Local Area Augmentation System (LAAS) andvarious European differential GPS networks. Even greater accuracy can beachieved by measuring GNSS carrier phase using so-called real timekinematic (RTK) techniques involving a nearby base station located at asurveyed position. RTK allows centimeter-level positioning, forexample.)

Tractor and implement paths may coincide from time to time, but ingeneral they are different. Curved paths offer a common scenario whichillustrates relationships between implement and tractor paths. A farmertypically begins a field operation by towing an implement along an edgeof the field. This first pass is done under manual control, but anautopilot records the implement path. In FIG. 2, for example, tractor200 pulls implement 210 along path 220. The implement path has curvesbecause the edge of the field may be curved or the farmer may have hadto maneuver to avoid obstacles such as trees or rocks. After the firstpass, the autopilot calculates a set of implement paths that willcomplete the job of covering the field with minimum overlap.

Equidistant swath layout, illustrated in FIG. 2, is one way toaccomplish this goal. Given an implement width, W, and a first path 220,the autopilot calculates a second path 230 that is W away from the firstpath. Equidistant swaths account for the implement width beingperpendicular to the direction of travel. In equidistant swath layout,convex curves such as 225 have increasing radii of curvature insubsequent swaths while concave curves such as 235 have decreasing radiiof curvature. To keep all parts of an implement moving forward, anequidistant swath layout method should enforce a minimum radius ofcurvature greater than or equal to W/2.

Anyone who has watched an eighteen wheeler make a turn at anintersection knows that the truck driver drives his cab out into themiddle of the intersection before making a wide turn. The rear wheels ofthe trailer take a much different path, hugging the inside of the turnand sometimes cutting the corner too much and jumping up onto the curb.If it were allowed to drive a truck, a conventional farm tractorautopilot would make the opposite mistake, rear wheels too far away fromthe corner, because the conventional autopilot does not anticipateturns. It only reacts to deviations of the implement (i.e. the trailer)from the implement's desired path and so it would not start makingcorrections until the rear wheels started to miss the turn. Back on thefarm, an autopilot that has laid out a set of equidistant swaths to worka field has in its memory knowledge of all the turns that an implementwill have to make. Autopilot path adjustment systems and methodsdescribed herein use knowledge of future implement turns to calculate anadjusted path for a tractor to follow so that the implement follows itsdesired path as closely as possible. The advanced autopilot is like avery good truck driver that sees a turn coming and drives his cab beyonda turning lane so that the rear wheels of the truck follow the laneperfectly.

FIGS. 3 and 4 illustrate basic and advanced curved-path adjustments,respectively. In FIG. 3, tractor 300 pulls implement 310 along desiredimplement path 320. The distance from the tractor reference point at themiddle of its rear axle to the hitch is H. The distance from the hitchto the implement reference point is L. When H is less than L (as itpractically always is) the implement tends to drift toward the inside ofturns. For the short segment shown in the figure, desired implement path320 may be approximated as an arc of a circle having radius R_(I). Thetractor follows an arc 330 having a greater radius of curvature,R_(T)=√{square root over (R_(I) ²−H²+L²)} to keep the implement on itsdesired path.

A basic method of predictive path adjustment, therefore, is to examinecurves in a desired implement path and at each curve add an offsetδR=R_(T)−R_(I) (toward the outside of the curve) to a feedback autopilotcross-track error set-point. This is what is done in some conventionalsystems. This method is better than doing nothing, but it has somedeficiencies: it does not take direction of travel into account; itadjusts cross-track error, but not tractor heading; and, it leads tosharp corrections at the beginning and end of turns.

FIGS. 4A and 4B illustrate an advanced curved-path adjustment. In FIG.4A, implement 410 is pulled along desired implement path 420. Thedistance from the implement reference point (x(u), y(u)) to its hitchpoint (x_(L), y_(L)) is L. A tractor, attached to the implement via ahitch, is omitted from FIG. 4A for clarity. An advanced method ofpredictive path adjustment first finds the path that the hitch musttravel to keep the implement on its desired path. For every point (x(u),y(u)) on the implement desired path, the corresponding point on thehitch path is given by: (x_(L), y_(L))=(x(u)+Δx, y(u)+Δy) where

${{\Delta \; x} = \frac{L}{\sqrt{1 + m}}},{{\Delta \; y} = \frac{m\; L}{\sqrt{1 + m}}}$and $m = {\frac{y}{u}/\frac{x}{u}}$

is the Cartesian slope of the desired implement path at (x(u), y(u)).(We use the phrase “Cartesian slope” to mean the derivative of afunction, e.g.

$\frac{y}{x^{\prime}}$

in a plane. “Cartesian slope” distinguishes this concept from hill“slope” which is terrain angled with respect to horizontal.)

In some cases, H<<L, and the hitch path is a close approximation to thetractor path. In general, however, a separate tractor path is calculatedstarting from the hitch path. The “tractor path” refers to the pathtaken by the control point of the towing vehicle. For a conventionaltractor the control point may be the midpoint of the rear axle, forexample. It may be the midpoint of the fixed axle in other wheeledvehicles, the center of vehicle rotation and/or the midpoint of atracked vehicle's track footprint.

FIG. 4B illustrates one technique for finding a tractor path from ahitch path. This technique relies on the assumption that the hitch pathcan be adequately approximated as circular for an along-track lengthequal to (or greater than) the hitch length. In FIG. 4B implement 440 ispulled along implement path 445 by hitch 460 that travels along hitchpath 465. The hitch is fixed to tractor 450 which travels along tractorpath 455. In FIG. 4B, hitch path 465 is circular for at least the lengthH that separates hitch point 460 from the control point of tractor 450.The radius of curvature of the hitch path is R_(H) as indicated in thefigure. The hitch extends from the rear of the tractor at right anglesto the tractor's rear axle and the tractor's heading is tangent to thetractor path. Given these constraints, a point on the tractor path maybe found from a corresponding point on the hitch path. As shown in thefigure, a tractor path point is located a distance H away from thecorresponding hitch path point at an angle

$\theta = {\sin^{- 1}\left( \frac{H}{R_{H}} \right)}$

away from the tangent to the hitch path.

R_(H) may change from point to point along the hitch path. If R_(H)changes quickly, so that the hitch path is not circular over the hitchlength, then the tractor path may be found by iterative solution oversegments shorter than H. Numerical techniques may be used calculateiterative solutions for the tractor path or even for an arbitrary chainof towed vehicles.

Returning for a moment to the tractor-trailer truck example, tractorpath 455 (or hitch path 430 in cases where H<<L) corresponds to the paththat the cab of a large truck takes around a corner, while implementpath 445 (or 420) corresponds to the path taken by the rear wheels ofthe trailer. In contrast to the basic method described in connectionwith FIG. 3, the advanced method does account for direction of travel;it produces a tractor path from which tractor desired heading may bederived; and, it leads to smooth implement transitions at the beginningand end of turns.

Given a desired implement path (from an equidistant swath layout, forexample), the advanced method described above leads to a desired tractorpath. The autopilot controlling the tractor therefore keeps track of twopaths: the desired implement path and the corresponding calculatedtractor path. In the context of this disclosure, the tractor path issaid to be “adjusted from the desired implement path.” The adjustment ispredictive in that it is performed before the tractor and implementactually travel on their respective paths.

If the tractor follows the calculated tractor path, then the implementwill follow the desired implement path; however, disturbances such asbumpy ground, steering errors, model errors and random effectsinevitably lead the implement astray. Therefore a feedback system, suchas that described in “Automatic Control of Passive, Towed Implements”(U.S. patent application Ser. No. 12/244,198, filed on Oct. 2, 2008),may be used to make the implement follow its path precisely. Predictiveadjustment of the tractor path makes the feedback controller's jobeasier: the effects of known influences (curves) are anticipated so thecontroller need only react to unpredictable, but small, disturbances.

An autopilot can make predictive adjustments to a tractor's pathwhenever the implement's desired path is known in advance. Given aseries of implement swaths covering a field, the autopilot may calculatea corresponding tractor path for the whole field quickly. However,because the calculation is such a simple exercise for a modernmicroprocessor it is not necessary to work very far ahead. As discussedabove, a hitch path point may be calculated from an implement path pointgiven the Cartesian slope of the implement path at that point and theimplement length, L. If the implement path is represented by a series ofdiscrete points fit by, e.g. a cubic spline, then the Cartesian slope ateach point depends only on the position of the point itself and thepositions of the nearest points ahead and behind on the path. Similarly,a tractor path can be calculated point by point from the hitch pathgiven the Cartesian slope at the corresponding hitch path point and thehitch length H. Thus a tractor path can be calculated point by point asneeded. On the other hand, one reason to calculate a complete tractorpath is to check for conflicts. A tractor may not be able to pull animplement right to the edge of a field if doing so would require thetractor go outside the field, through a fence, into a ditch, etc.

As briefly mentioned above, H<<L in some cases. It is even possible forH=0; i.e. the implement is connected to the tractor at the tractor'scontrol point. Another scenario, that applies, for example, toself-propelled sprayers, is L=0. When L=0, the implement is a fixedextension of the tractor. (Fixed extensions include implements rigidlyattached by three-point connections.) On a self-propelled sprayer, spraybooms extend out from the spray truck, but their spatial orientation isfixed with respect to the truck. In contrast, farm implements towed froma hitch (L≠0) pivot from the hitch point. Thus, for purposes of thisdisclosure, a “hitch” is a fixed extension from a vehicle. H is thedistance from the vehicle control point to: (1) a hitch point that pullsan implement (L≠0); or, (2) a fixed extension from the vehicle (L=0),e.g. a spray nozzle on a fixed spray boom.

As we have seen, a curve is an example of a predictable influence whoseeffect can be anticipated. Another example is a slope. When a tractorguided by a feedback autopilot pulls an implement across a hill, theimplement drifts downhill. Sensing the positioning error (implementdownhill from desired path), a feedback autopilot guides the tractoruphill to compensate. On a straight path on a constant slope, thetractor and implement reach a steady state with the tractor path uphilland parallel to the implement path, the tractor path being offset fromthe desired implement path just enough that the implement stays ontrack. A feedback autopilot can only react to implement path errors,however. There is inevitably some time during which the implement driftsdownhill while waiting for the autopilot-controlled tractor to makesufficient corrections. The sloped-path adjustment of FIG. 5 eliminatesthe wait by anticipating the effect of slopes and compensatingaccordingly.

FIGS. 5A and 5B illustrate a sloped-path adjustment. In FIG. 5 tractor500 pulls implement 510 along desired implement path 520. The tractoraccomplishes this task by driving on tractor path 530 uphill from theimplement path by an amount “d”. The distance between the implementreference point and its hitch is “L” as shown in FIG. 5A. FIG. 5B showsthe sloped ground 540 upon which the tractor and implement travel. Theangle of the slope with respect to horizontal is θ.

A simplified, but illustrative, model of an implement is a block that isdragged behind a tractor. The force required to move it is a fraction,μ, of its weight. μ may be thought of as a coefficient of friction; itis zero if the block is resting on a frictionless surface but it may begreater than one if the block has a good grip on the surface below it.The amount, d, that an implement sags downhill, or equivalently theamount by which a tractor must offset its path uphill to keep theimplement on track is given by:

$\frac{d}{L} = {\frac{\frac{1}{\mu}\tan \; \theta}{\sqrt{1 + \left( {\frac{1}{\mu}\tan \; \theta} \right)^{2}}}.}$

This relationship is illustrated in FIGS. 6A and 6B which are graphs ofimplement sag (d/L) versus slope and sag versus coefficient of friction,respectively. As an example, FIG. 6A shows that when μ=0.8, a ten degreeslope will cause an implement (modeled as a block) to sag downhill byabout 20% of L. FIG. 6B shows that the sag on a ten degree hill can bereduced to about 10% of L if μ can be increased to around 1.5 or more.d/L is an approximately linear function of μ or θ for typical values ofthose variables. When friction is very small, implement sag approachesimplement length.

In practice μ may be a complicated, nonlinear function of soilconditions, implement details and hill slope, θ. Further, μ for typicalfarm implements is strongly dependent on direction; it is a lot easierto pull a drawn chisel plow forward than sideways, for example.

Estimating μ from first principles is most likely futile, but linear orhigher order models for implement sag as a function of hill slope can bedetermined by least-squares approximation of recorded sag vs. slopedata. The observed sag of an implement in known soil conditions on aknown slope can be used to create such an empirical model. The model maythen be used to predict downhill sag of the same implement in the samesoil conditions on a different slope.

An autopilot has advance knowledge of the desired path of an implementas discussed above in connection with path adjustments to anticipate theeffect of curves. The autopilot may also have knowledge of the terrainover which a path runs. This knowledge may take the form of digitaltopographic and/or soil condition maps, for example. Topographic and/orsoil condition data may be stored in memory in an autopilot (e.g.microprocessor RAM, flash drive, etc.) or it may be supplied as neededby wireless link (e.g. WiFi, other radio services, etc.). Furthermore,an autopilot's roll sensor may be used to measure terrain slope and,since the tractor is ahead of the implement by distance L, provideadvance warning of slope changes.

Terrain and/or soil condition data allow an autopilot to make predictivetractor path adjustments on slopes. A tractor path may be calculated asan uphill offset from an implement path where the amount of offsetdepends on terrain slope, soil condition and implement type. Anautopilot may also record the amount of downhill sag for a specificimplement, slope and soil condition, and use least-squares estimation tomodel the relationship between sag and slope. Uphill tractor offset maythen be calculated based on the recent history of offsets needed tocompensate for various slopes. The autopilot may continuously update itsmodel as field conditions change. Accumulated data leads to improvedpredictions for the same implement on different slopes and/or differentsoil conditions. Soil condition maps may be generated from implement sagdata when terrain slope is known.

An autopilot can anticipate hills from map data, roll data recorded fromprevious trips over the same ground or roll sensor input. If the tractorfollows the predicted uphill path, then the implement will follow itsintended path in the absence of disturbances. Since disturbances cannever be eliminated, a feedback system, such as that described in“Automatic Control of Passive, Towed Implements” (U.S. patentapplication Ser. No. 12/244,198, filed on Oct. 2, 2008), may be used tomake the implement follow its path precisely. Predictive adjustment ofthe tractor path makes the feedback controller's job easier: the effectsof known influences (slopes) are anticipated so the controller need onlyreact to unpredictable, but small, disturbances. In addition to bumpsand other noise, disturbances include deviations of actual terrain slopefrom its expected value.

Wind is a third example of an influence whose effect may be predicted.When a tractor or spray truck sprays chemicals on a field on a windyday, the spray tends to drift downwind before landing on crops or groundas intended. This effect can be compensated for by adjusting the tractoror spray truck's path upwind.

FIGS. 7A and 7B illustrate a crosswind-path adjustment. In FIG. 7Atractor or spray truck 700 sprays chemicals from spray boom 710 inno-wind conditions. Spray 740 is evenly distributed on either side ofintended spray path 720. In FIG. 7B the tractor or spray truck sprayschemicals in crosswind conditions. Because of the wind, the tractorfollows path 730, upwind of intended spray path 720, so that spray 750lands evenly distributed on either side of the intended spray path. Ofcourse, not all winds are direct crosswinds. A convenientsimplification, therefore, is to consider only the crosswind componentof a quartering wind. In general, however, the effect of wind in anydirection may be taken into account.

A tractor or spray truck may detect apparent wind speed and directionwith an onboard anemometer and wind vane. This information may becombined with an autopilot's knowledge of tractor speed and direction tocompute the true wind speed and direction. Alternatively, wind directionand speed may be measured remotely, at the edge of a field or at anearby crop duster strip, for example, and the data may be sent to thetractor via radio.

Onboard and remote wind sensing may be combined. Suppose, for example,that the local weather forecast calls for wind from the east at 20knots. An autopilot may use that information to lay out a series ofswaths to spray a field. The tractor path is upwind (east) of where itwould be in zero wind. If an onboard anemometer measures the actual windto be from the east at 23 knots, then the autopilot may make anadjustment to take into account the additional, unpredicted wind.

The amount of offset (“d” in FIG. 7B) required for a particular sprayapparatus versus wind speed may be determined empirically and theresults stored in memory. Spray droplet size may have a profound effecton drift distance. Unlike the influences of curves and hills discussedabove, the effect of wind on spraying operations isn't usually monitoredautomatically. The tractor or spray truck's path is not under feedbackcontrol with respect to actual spray results. The benefit of upwind pathadjustment depends on the accuracy of empirical calibration data.Nonetheless the capability of adjusting upwind offset automatically aswind speed changes makes crosswind spray operations easier for sprayoperators as such changes are not necessarily readily apparent in atruck or tractor cab.

FIG. 8 provides an overview of an agricultural autopilot that makespredictive path adjustments for curves, hills, wind or other knowninfluences. In FIG. 8, autopilot 800 receives input from sensors 810 andexternal data 820; the autopilot sends output to actuators 830.Autopilot 800 may include a microprocessor, user interface, tractorcontrol system, terrain map, path memory and path adjustment capabilityas well as other components. Sensors 810 may include tractor position,speed, acceleration and attitude sensors, implement position, speed,acceleration and attitude sensors, apparent wind angle and speed sensorsas well as other components. External data 820 may include maps ofterrain, soil condition, or other variables, weather data, and otherdata. Actuators 830 may include tractor steering angle and speedactuator, and other actuators.

In autopilot 800 a user interface may include a display screen and realor virtual buttons for user input. An example of a tractor controlsystem that may be included in the autopilot is a control system such asthat described in “Automatic Control of Passive, Towed Implements” (U.S.patent application Ser. No. 12/244,198, filed on Oct. 2, 2008). Pathadjustment calculations are performed within the autopilot by amicroprocessor according to the methods described above; path memorystores calculated path data.

Sensors 810 include GNSS receivers, accelerometers, gyroscopes and othersensors, many of which are parts of conventional autopilot and implementtracking systems. Apparent wind angle and speed may be measured by awind vane and anemometer. True wind angle and speed, as measured byexternal instruments may be part of external data 820.

FIG. 9 is a flow chart for a predictive path adjustment method. Steps inthe flow chart of FIG. 9 are optional unless otherwise stated. Thestarting point for the method in FIG. 9 is step 910, determine (orprovide) a desired path for implement. The desired implement path may bethe result of an equidistant swath layout calculated by an autopilot; itmay be a layout designed externally (e.g. in a computer aided farmingsoftware suite) and loaded into an autopilot as a series of path points;or it may be determined by another method.

Step 920 is using known data to determine required tractor path inadvance. Known data 940 pertains to predictable influences such asimplement path curves, slopes, soil conditions, wind and other knownfactors. The required tractor path is the path that, in the absence ofdisturbances, compensates for the effects of known influences such thatthe implement follows its desired path.

Because disturbances can never be eliminated, step 930 is guiding thetractor along the tractor path optionally using feedback to maintain theimplement on the implement path. Although feedback is desirable forimplement control, it is not required and in some cases it is not easyto implement. For example, it is not easy to continuously monitor theeffect of wind on a spray pattern.

Step 930 further includes using live measurements 950 to adjust afeedback control system on the fly. These live measurements help reducethe effects of unanticipated changes in known influences 940. Considerfor example a path that traverses a slope. An autopilot calculates atractor path that will compensate for the effect of the expected slope.When the tractor arrives at the slope, it may turn out that the actualslope is different from expected. A control system will eventuallycompensate for this difference using feedback from the reportedimplement position just as it would without advance slope information.Better performance is obtained, however, if the difference betweenexpected and actual conditions is used to adjust a feedback controlsystem, e.g. by changing a cross-track error set point, on the fly.

Finally data, such as actual tractor and implement paths, may berecorded for future use. Recorded data is often the most accurate andprecise source of terrain information.

The systems and methods described above improve implement (and spray)control performance by anticipating the effects of influences such ascurves, slopes, soil changes, wind, etc. This improved controlcontributes to more precise and efficient farming.

The above description of the disclosed embodiments is provided to enableany person skilled in the art to make or use the disclosure. Variousmodifications to these embodiments will be readily apparent to thoseskilled in the art, and the principles defined herein may be applied toother embodiments without departing from the scope of the disclosure.Thus, the disclosure is not intended to be limited to the embodimentsshown herein but is to be accorded the widest scope consistent with theprinciples and novel features disclosed herein.

What is claimed is:
 1. A method for improving agricultural sprayaccuracy in crosswinds comprising: providing a desired spray path, thepath represented by a set of spray path points; constructing a tractorpath represented by a set of tractor path points, each one determined byadjusting a corresponding spray path point upwind by an amount that is afunction of the true wind speed at the tractor; guiding a tractor alongthe tractor path as it sprays an agricultural chemical.
 2. The method ofclaim 1, the function of the true wind speed being the square of thetrue wind speed.
 3. The method of claim 1, the function beingrepresented by a table of tractor offsets versus wind speed.
 4. A systemfor guiding an agricultural spray vehicle comprising: vehicle positionand speed sensors for sending vehicle position and speed information toan agricultural autopilot; wind angle and speed sensors for sending windangle and speed information to the autopilot; and, actuators foradjusting vehicle steering angle in response to autopilot commands;wherein, the autopilot: a) comprises a memory that stores a spray paththat is represented by a set of spray path points; b) calculates avehicle path, each point of which is determined by wind direction andspeed, and a corresponding point on the spray path; c) guides thevehicle along the vehicle path using the vehicle position and speedinformation.
 5. The system of claim 4 wherein the vehicle position andspeed sensors are based on a GNSS receiver.
 6. The system of claim 4wherein the wind angle and speed sensors are mounted on the vehicle andthe autopilot further calculates the true wind angle and speed based onapparent wind angle and speed measured by the wind angle and speedsensors.
 7. The system of claim 4 wherein the wind angle and speedsensors are mounted at an external fixed location and send wind angleand speed information to the autopilot via radio.